Larger tires and wheels back spacing help

[1963chev]

I have 235 R75 15s on the original rims on my truck and in the rear they are centered almost exactly in the wheel weld between the fenders and the bed. I like the setup as far as steering and handling but need tires and want new rims (see pic) so I am moving to 16x8 rims and 255 R65 16s. What backspacing should I choose? 4.0 because I am centered now? Thanks for any help or advice.

[PGSigns]

I run 15X8 with 4" of back spacing on the rear of my 66 fleet with 275's with no issues.
Jimmy

[1963chev]

Thanks Jimmy. I have done so much customization like 72 rear end and 71 front end, lowered 2/3, ps, etc that I can't really make comparisons to stock trucks. What I need to find out is how to match the back spacing of what I have on the truck now. I just pulled one of the 15x7 rims off, measured, and found that with the rim face up, it measures 4 inches from the floor to the bolt holes. With the rim face down, it measures 3 inches from the floor to the bolt holes. So, does anyone know what back spacing that is? If I go to 16x8, what back spacing should I get to match what I have now? Thanks for any help

[jtrichard]

IF you are well centered now with 4" back spacing you will want 3-1/2 back and 2-1/2 on the face .... 1" wider wheel 7" now? vs 8" new

[1963chev]

Thanks Jimmy. I have done so much customization like 72 rear end and 71 front end, lowered 2/3, ps, etc that I can't really make comparisons to stock trucks. What I need to find out is how to match the back spacing of what I have on the truck now. I just pulled one of the 15x7 rims off, measured, and found that with the rim face up, it measures 4 inches from the floor to the bolt holes. With the rim face down, it measures 3 inches from the floor to the bolt holes. So, does anyone know what back spacing that is? If I go to 16x8, what back spacing should I get to match what I have now? Thanks for any help

[1963chev]

Yes I am well centered now with the 7 inch rims but I don't know their back spacing. With the measurements I gave, can you tell me what the back spacing is?

[Captainfab]

Wheel backspacing is measured from the inside of the wheel where it bolts to the drum/axle flange to the edge of the wheel. This does not include the tire. If you measured correctly it sounds like your current wheels have a 4" back spacing.

[1963chev]

Thank you for the help, that's a lot easier than I was thinking. If I understand, the back spacing is just the measurement from the bolt holes to the BACK (inner) edge of the rim. I am at 4.0 back spacing with the 7 inch wide rims I have now. Since I am moving to an 8 inch wide rim is it correct that I need to get 4.5 back spacing to keep the wheel and tire centered where they are now?

[jtrichard]

OPPS had my math bassackwards above yes it would be 4-1/2" backspace and 3-1/2 on the face with an 8" wide wheel

[The Rocknrod]

The Wheel Smith has a great backspace diagram and you can download and print their sheet to work with.
http://www.thewheelsmith.net/WheelGeometry.html
Sheet:
http://www.thewheelsmith.net/images/...0spec-form.pdf

[lolife99]

Originally Posted by jtrichard

OPPS had my math bassackwards above yes it would be 4-1/2" backspace and 3-1/2 on the face with an 8" wide wheel

Remember most wheels measure 1" wider than their advertised width.
This is because wheels are measured at the bead surface,... NOT the actual outside-to-outside width.
Look at a bare rim and you will see what I'm talking about.
You need to account for this when measuring backspacing.

[1963chev]

Man o man, thank you all for taking the time to help. I was about to make a mistake but I was saved!

[1963chev]

So the move from 15x7 rims with 235 75R15 tires to 16x8 rims and 255 65R16 went great as far as fit, handling, and ride quality but I notice my truck is not as zippy off the line and works harder pushing the bigger tires around. I'm surprised. Any thoughts?

[The Rocknrod]

Originally Posted by 1963chev

So the move from 15x7 rims with 235 75R15 tires to 16x8 rims and 255 65R16 went great as far as fit, handling, and ride quality but I notice my truck is not as zippy off the line and works harder pushing the bigger tires around. I'm surprised. Any thoughts?

Tire calculator shows a .17" difference in diameter, so I don't get it.
Your revs per mile will be only 4.16 revs less.
Your speedo will read .39mph difference.
http://www.americastire.com/dtcs/infoTireMath.do

[1963chev]

Originally Posted by The Rocknrod

Tire calculator shows a .17" difference in diameter, so I don't get it.
Your revs per mile will be only 4.16 revs less.
Your speedo will read .39mph difference.
http://www.americastire.com/dtcs/infoTireMath.do

Thanks for the link, Rock. Does the weight of the bigger rims and wider tires come into play? Do you ever adjust air pressure? I'm running 35 psi.

[chevy_mike]

Originally Posted by 1963chev

Thanks for the link, Rock. Does the weight of the bigger rims and wider tires come into play? Do you ever adjust air pressure? I'm running 35 psi.

Yes, rotational load plays a very big part. The more the wheel/tire combo weighs, the slower it will be to accelerate. I had this happen on my '05 Ram. Went to 20" factory wheels and tires that were the same diameter as the 17" ones. My acceleration is much slower than it was. I found out that the factory would use 3.92 gears with 20" wheels versus the 3.55 gears with the 17" wheels. This was to help compensate for the extra rotational load.

[The Rocknrod]

Originally Posted by chevy_mike

Yes, rotational load plays a very big part. The more the wheel/tire combo weighs, the slower it will be to accelerate. I had this happen on my '05 Ram. Went to 20" factory wheels and tires that were the same diameter as the 17" ones. My acceleration is much slower than it was. I found out that the factory would use 3.92 gears with 20" wheels versus the 3.55 gears with the 17" wheels. This was to help compensate for the extra rotational load.

Interesting. I am considering 17" wheels and already purchased 3:42 rear gears with a 200R4 trans which has a lower gear slightly than the 700R. Hope it works.
Could use 225/65R17 or the like to keep the diameter down.
Here is a great page on tire sizes: https://tiresize.com/height-chart/
And a great page on gear ratios that includes your transmission ratio: http://www.csgnetwork.com/multirpmcalc.html

[LostMy65]

Originally Posted by chevy_mike

Yes, rotational load plays a very big part. The more the wheel/tire combo weighs, the slower it will be to accelerate. I had this happen on my '05 Ram. Went to 20" factory wheels and tires that were the same diameter as the 17" ones. My acceleration is much slower than it was. I found out that the factory would use 3.92 gears with 20" wheels versus the 3.55 gears with the 17" wheels. This was to help compensate for the extra rotational load.

Interesting info.
Does this go for wider wheels also?

[chevy_mike]

Originally Posted by LostMy65

Interesting info.
Does this go for wider wheels also?

Not as much of a factor if the same diameter wheel and tire, unless the wider tire is a large amount heavier then what was on before. You will increase the rolling resistance a little but that is very minor.

The rotation forces come into play when larger diameter wheels are used, which can end up weighing more than the smaller one. If everything is equal (i.e. tire overall diameter, total wheel/tire weight) there will not be a difference.